A reduction in the dielectric constant or an interlayer dielectric film is an important problem for an increase in a speed and a reduction in a consumed power in a semiconductor device. Various devises have been made in order to reduce the dielectric constant. For a conventional semiconductor device, there have been proposed methods of:
(1) adding fluorine to a silica film to be an inorganic insulating film;
(2) forming an organic insulating material having a low dielectric constant as a parent material; and
(3) intentionally forming a porous film.
In the case of the method (1), however, the moisture resistance of the insulating film is deteriorated. For this reason, the fluorine can be added in an element ratio or several % at most. Therefore, there is a problem in that a dielectric constant can be reduced by 10% to 15% at most as compared with a conventional silica type interlayer dielectric film.
In the case of the method (2), moreover, there is a problem in that a heat resistance and a mechanical strength are reduced more considerably than those of the conventional silica type interlayer dielectric film because of the organic material, resulting in a deterioration in the reliability of a semiconductor device.
In the case or the method (3), furthermore, the mechanical strength of the interlayer dielectric film is remarkably reduced because a porous structure is random, and there is no CMP (chemical mechanical polishing) resistance. For this reason, the film is apt to be broken in packaging, causing a deterioration in the reliability of the semiconductor device.
In many cases, moreover, the porous structure is not closed. If the porous structure is not closed, the moisture resistance of the interlayer dielectric film is remarkably reduced, resulting in a deterioration in the reliability of the semiconductor device.
In the conventional insulating film, thus, there is a problem in that the dielectric constant cannot be reduced sufficiently, and furthermore, a sufficient mechanical strength cannot be obtained.
Therefore, the inventors have proposed a method of forming a pure mesoporous silica thin film by dissolving, in a solvent, a surfactant, a silica derivative and an acid catalyst in a desirable mole ratio, preparing a precursor solution in a mixing vessel, applying the precursor solution thus prepared onto a substrate, polymerizing the silica derivative through hydrolysis (polycondensation reaction) (a precrosslinking step), forming a mesoporous silica thin film including a cavity having, as a template, the periodic self-assembly or the surfactant, and thermally decomposing and removing the surfactant of the template completely at a calcining step.
At this time, the substrate is exposed to a silica derivative atmosphere and is heated with the supply of the silica derivative prior to the calcining. Consequently, the shrinkage of the film is prevented from being caused by the hydrolysis, and the cavity is not broken but maintained as it is so that a mesoporous silica thin film having, as a template, the rigid self-assembly of the surfactant is obtained. Furthermore, the surfactant of the template is thermally decomposed and removed completely at the calcining step so that a pure mesoporous silica thin film is obtained.
Thus, it is possible to provide a dielectric thin film having a very excellent controllability, a great mechanical strength and a very low dielectric constant.
However, the strength and the dielectric constant are not sufficient and a further enhancement in characteristics has been required.